March 2013 Archives

In order to elaborate further on the differentiation of species and organisms within species after the phylotypic stage and its relation to Hox genes, I thought it might be useful to discuss the evolution of wing number and form. In Sean B. Carroll's book, Endless Forms Most Beautiful, he describes this process by using the gill-to-wing theory. The gill-to-wing theory has much evidence that supports the evolution from crustacean gill branches to insect wings. Of course, there are many transitional evolutionary steps that this change required. For example, a mayfly nymph, a primitive winged adult, and a modern lepidopteran all have very different types and numbers of wings. Specifically, the mayfly nymph does not have any wings on T1, whereas the primitive winged adult has reduced T1 structures and no wings on the abdomen.

So, how does this differentiation of wing number and type occur across these species? According to Carroll, it is much related to my previous post that related this specilaization to Hox genes. In fact, the addition of more Hox gene binding sites allowed for the promotion and differentiation of wing development. Therefore, it is evident that Hox genes and their binding sites play a major role in determining the structures and forms of unique species.

In class this week, we discussed the idea of phylotypic stage. In many previous courses, including biology and psychology courses, I was always taught that early embryos of separate species look extremely similar. While this is partly true, the artistic representation (Image 1) that has always been displayed in class is exaggerated. The 'partly true' portion of this phylotypic stage, however, is quite interesting to me. You can see a realistic version of the phylotypic stage in Image 2.

During the phylotypic stage, structures are similar across very different species including the notochord, somites, and neural tube. So, how do such similar structures develop into such unique organisms? Well, it is theorized that as the phylotypic stage progresses, Hox genes are expressed which specify positional identity. The pattern of Hox gene expression is very different across species, which allows for specialized body plans and unique organisms. Therefore, the phylotypic stage tapers off as species become less similar, much like the hourglass model seen in Image 3.

For our next lab, we will be taking a progressive set of pictures that will be arranged into a movie. The theme: something interesting in drosophila development! I thought it would be helpful to look up the development of drosophila in order to get a better idea of what I want to focus on (all dependent factors aside. i.e. time). I realize that while I might like a certain part of development more than another, it might be near impossible to capture that moment in time. I think it would be interesting to look at the gradual development from a pupa to an adult, giving rise to legs and wings. However, since they encapsulate themselves, it may be impossible to see this transformation. Therefore, I would like to try to see when the adult drosophila emerges from the pupa into a fully formed organism with wings and all. Although it may be hard to identify when the transformed adult will emerge from the pupa, I think I will be able to monitor it enough to get a good idea with the help of Professor Myers. I am hoping that it will look much like the video linked below!Drosophila Video- Pupa to Adult

This is a response/expansion to a post written about gynandromorphy by a student in my developmental biology class, Miles. It may be useful to read this post prior to reading the following post.

Gynandromorphy occurs when an organism displays both female (gyne) and male (andro) physical characteristics (morph) due to sexual dimorphism. In the photo of the gynandromorph lobster, it appears as though it is bilaterally symmetrical. However, this is not always the case. In fact, some gynandromorph organisms display a mosaic pattern in which the two sexes are not differentiated as clearly. The pattern of gynandromorphy is dependent on the determination pattern of cells during division. This disorder is a result of an early event in development, specifically mitosis, which does not split the sex chromosomes in a typical manner. Although I was curious as to whether these organisms can reproduce, I couldn't find any information on the topic. However, many articles related this disorder to hermaphrodites and intersex individuals of the human species. In these cases, most individuals are sterile as a result of their disorder. With most individuals, the reproductive organs are incomplete and can, therefore, not reproduce. It may be possible that an individual with gynandromorphy or hermaphroditism with complete organs could reproduce but I do not know nearly enough about the subject to speculate.

Learning about genes, proteins, switches, 'genetic tool kits,' and the like has been particularly interesting. However, the part of this course that has been distinctively exciting is how these aspect of development are connected to evolution. As described in Endless Forms Most Beautiful by Sean B. Carroll, wing development sheds some light on powerful evidence of evolutionary development. From gills to wings, a few proteins have been identified as necessary components for this type of development. Specifically, Apterous and Nubbin have been found to be selectively expressed in areas where wing and gill development take place (i.e. the respiratory lobe in crustaceans).

It is evident that these homologous parts are significant in the study of evolutionary development. In fact, this particular case can tell us a lot about evolution of many species, including humans. Relating to Hox proteins and genetic switches, the repression or expression of specific products or parts, such as wings of insects, can be traced back. These genetic switches were involved in the development of signature sequences for Hox proteins that can result in either the 'turning on' or 'turning off' of particular segments. Therefore, differences in wing expression among separate species can be traced back to common Hox protein and the expression or repression of sequences and proteins.